The present invention relates to thermal storage systems, and particularly to a phase change thermal storage and heat transfer system useful for heating, air conditioning, and process cooling.
Thermal storage systems including latent heat storage systems based on clathrate hydrates have proven effective for storing thermal energy through the formation and decomposition of clathrate material. A clathrate is a compound formed by the inclusion of molecules of one kind in the cavities of the crystal lattice of another. This change of phase is a result of an enthalpy change that is commonly 100 to 144 BTUs per pound of clathrate material within the system.
Thermal storage systems using clathrates show great promise in facilitating efficient utilization of heating and cooling energy. In many countries, including the United States, electrical energy used for cooling is available at reduced rates during periods when electrical demand is low. Energy stored in phase change materials using such "off-peak" electricity can be released and used during peak periods, reducing overall energy costs.
The energy storage density of many clathrates is extremely high, assuming efficient conversion of the storage medium to clathrate. Therefore, it is possible for an efficient cooling system utilizing clathrate storage materials and a relatively small chiller operating in the charge mode for several hours to provide an instantaneous cooling capacity equivalent to that of much larger conventional air conditioning equipment. This size savings is a significant advantage, regardless of whether off-peak electrical energy is available at reduced rates.
In addition to storage for cooling or, in appropriate circumstances, heating, another major potential use for clathrate storage systems is in water purification. Clathrate crystal formation can occur at temperatures well above 32.degree. F., and the clathrate crystal contains only water and hydrate agent (usually a gas molecule). Impurities in the liquid water from which the clathrates are formed are not included in the crystalline structure. Thus, impure water may be used to form clathrates, the clathrate crystals may be separated from the remaining water, the separated crystals may be melted to provide pure water and hydrate agent, and the hydrate agent may be recycled to form additional clathrate.
One practical difficulty with existing water purification systems utilizing clathrates is that impurities are sometimes entrapped in the large crystals that form when the unit crystals agglomerate.
One approach for utilizing phase change storage materials is described in U.S. Pat. No. 4,696,338 to Jensen, et al., and assigned to the assignee of the present invention. This system employs direct contact heat exchange for the purposes of agitation and the transfer of heat into and out of the system. Typically, systems employing this type of heat exchange, in which heat exchange takes place within the storage medium, require the use of custom-made heat exchanger coils and other components, making the system relatively expensive to manufacture.
Other systems employ heat exchangers inside the storage tank. These systems tend to create large crystalline masses within the tank. In this case, when it is desired to cool a system, warm liquid is sent through a heating/cooling coil (or other heat exchanger), which melts the crystalline mass surrounding the coil and thereby cools the fluid therein. As the frozen storage material around the coil melts, a warm water pocket forms around the coil, surrounded by the remaining crystalline mass. Thus, after a certain amount of time, cooling occurs primarily through natural convection (as opposed to forced convection and conduction), which in a large system is a time consuming, inefficient method of transferring heat.
Similarly, when charging the system, solid crystalline masses tend to form in layers which emanate from the cooling coil. Accordingly, the outer layers of the crystal will tend to form much more slowly than the inner layers, because heat transfer by conduction through the crystalline mass is relatively inefficient.
Another approach, described in U.S. Pat. No. 4,051,888 to Yamada, et al., is a system which uses external heat exchange. A major disadvantage of a system of this type is the tendency to form large clathrate crystals. Larger crystals, because of their size, are very difficult to transfer through the heat exchanger. The resultant poor flow characteristics, caused by the formation of large clathrate crystals, greatly reduces the efficiency of the heat exchange process during the discharge cycle. Yamada, et al. describe a crystal crusher to deal with this problem. Additionally, the system described by Yamada, et al. is designed to supplement chilled water storage systems, and is thus content with low reaction yields.
Thus, while systems employing the formation and decomposition of clathrate material have been effectively used, obtaining complete reaction of water and hydrate agent during the charge cycle has proven difficult. Further, the formation of large crystalline masses within the storage tank of previous systems has inhibited effective and rapid heat transfer into and out of the storage system.